Generation of accurate 2-dimensional and 3-dimensional geological maps is essential for contemporary oil exploration in order for personnel such as geologists to identify geological formations which are likely to yield quantities of oil and/or gas which are profitable to extract. Generation of such geological maps is implemented using seismic apparatus. Each seismic apparatus comprises one or more seismic transmitters and one or more seismic receivers; optionally, the one or more seismic transmitters are also operable to function as one or more seismic receivers. The one or more seismic transmitters are operable to generate seismic waves in a ground formation to be mapped. Reflections of the seismic waves at various rock interfaces present in the ground formation, caused by seismic wave propagation impedance mismatch thereat, are received at the one or more seismic receivers to generate received signals; the impedance mismatch results, for example, as a consequence to spatial variations in rock density at the interfaces. The signals are digitized to generate seismic data. Subsequently, the seismic data is then provided to computing hardware provided with appropriate data processing software to isolate from:    (a) amplitudes of seismic signals received at the one or more seismic receivers, and also    (b) time of reception of the seismic signals,seismic components from specific regions of the aforesaid interfaces and thereby determine spatial positions within the map of the regions of the interfaces. The data processing software executes a series of one or more matrix computations in order to derive the geological maps, the computations often requiring considerable computation effort.
It is found in practice that such seismically-derived geological maps are not always entirely accurate on account of noise, ambiguity of reflected seismic components, faults in rock layers affecting seismic wave propagation therethrough or reflection thereat which contribute to cause inaccuracies. However, in contemporary oil exploration, well boreholes often need to be drilled with considerable accuracy, especially when such well boreholes include a first upper portion which is substantially vertical and a lower portion which is at least in part substantially horizontal and required to follow a specific layer of geological strata in which oil deposits are to be found. For example, it is not contemporarily uncommon to aim to drill boreholes substantially horizontally along a layer of strata which has a vertical height of 5 meters at an end of well borehole having a length of 10 km.
Improvements in contemporary directional drilling have allowed drillers to drill a well borehole in almost any orientation. The drillers are then confronted with a problem of navigating their well boreholes during drilling. As elucidated in the foregoing, oil is typically located in relative thin stratigraphic zones which represent a significant technical challenge for the aforesaid drillers to target precisely. Optionally, a driller is desirous to tap into a stratigraphic zone of interest with a well borehole that traverses inside the zone for an extended distance. For example, if a target zone has a true horizontal orientation, then the well borehole, when it penetrates into the zone, extends horizontal to remain within the zone.
Prior art only provides less accurate techniques for maintaining a well borehole inside an aforementioned target zone. The target zone is typically thousands of meters below the Earth's surface or ocean seabed and is, in many situations, only 3 to 10 meters thick. Furthermore, stratigraphic zones are typically inclined or dipped relative to a horizontal plane. Thus, the target zone is a difficult target in which to maintain the well borehole during drilling operations. A well borehole drilled pursuant to prior art techniques is susceptible to quickly exiting the zone because:    (a) the well borehole has a direction is not parallel to that of the zone; and    (b) a spatial region occupied by the zone is not known with certainty in seismically-derived stratigraphic maps.
In a published U.S. Pat. No. 5,311,951 (Kyte & Meehan), drilling techniques are described which allow a well borehole to be drilled downwardly, horizontally and upwardly. There is also elucidated a method of navigating a borehole drillable by utilizing directional drilling techniques. When implementing the method, a direction in which a borehole is drilled is determined relative to a stratigraphic target zone. By so navigating the borehole, it is feasible for the borehole to enter the target zone and extend inside the target zone. In order to navigate the borehole, an offset log of the borehole is obtained during drilling. Correlation points along the borehole are selected, for example by manual intervention or inspection. At each correlation point, a true stratigraphic depth of the borehole is computed from the offset log. Thus, knowing the true stratigraphic depth of the borehole allows the location of the target zone relative to the borehole to be determined, wherein the direction that the next segment of the borehole during drilling of the borehole is determined. The aforesaid U.S. patent (Kyte & Meethan) is concerned with a 2-dimension trajectory of a subterranean borehole.
Expressed more generally, the published U.S. Pat. No. 5,311,851 defines a method of determining the location of a borehole relative to strata in the Earth, wherein the method comprises steps of:    (a) providing information from the borehole, for example an offset log, such that the information characterizes the strata; and    (b) providing characterizing information of the strata from an offset location;    (c) comparing the characterizing information from the borehole relative to the characterizing information from the offset location to determine the location of the selected points along the borehole relative to the strata.
The method described in published U.S. Pat. No. 5,311,951 enables the borehole to be drilled with greater accuracy in respect of stratigraphic information. However, if the stratigraphic information is not accurate, the borehole cannot be drilled to an accuracy greater than that of the stratigraphic information. Such inaccuracy represents a technical problem which the present invention seeks to technically address. A conventional approach to improving stratigraphic information is to undertake more extensive seismographic measurements, for example from more surface locations; however, such measurements are subject to fundamental limitations in that ill-defined boundaries are not rendered better spatially defined in a stratigraphic map simply by collecting more seismic measurements confirming that the boundaries are ill-defined.